This invention relates to electrophotography and, more especially, to particulate electrophotographic toners and developers.
Electrophotographic imaging processes have been described extensively in patents and other literature. These processes have in common the forming of an electrostatic charge pattern on an insulating photoconductor. The pattern, or latent electrostatic image, is made visible by contact with a developer containing electrostatically charged toner powder. Several methods of dry development are available, including the well-known magnetic brush and cascade development methods.
Most dry developers are a mixture of toner particles and carrier particles. For magnetic brush development the latter can be a magnetic substance such as iron filings, powdered iron or iron oxide. For cascade development and other methods the carrier particles can be non-magnetic substances such as glass or ceramic beads. The toner particles become triboelectrically charged by frictional contact with the carrier particles. Then, when contacted with the oppositely charged image pattern on the photoconductor, they adhere to the charged areas and make the image visible. In well-known office copying machines the developed toner image is transferred from the photoconductor to a sheet of plain paper to which it is fixed by fusion or other known techniques.
While the polymer is the major component of the toner, certain addenda usually are dispersed in the polymer. These can include one or more colorants such as pigments and dyestuffs which make the developed charge pattern visible. Also desirable as addenda are ionic compounds which help to maitain a uniform, stable, high net electric charge on the triboelectrically charged toner particles. These compounds are known as charge control agents.
A variety of charge control agents for toner particles have been proposed. For example, Olson U.S. Pat. No. 3,647,695 describes an electrostatic toner containing a mono- or di-functional organic acid nigrosine salt which aids in providing a relatively high uniform net electrical charge on the toner particles. It has been found, however, that the nigrosine salts decrease the adhesion of the toner particles to a paper receiving sheet.
Greig U.S. Pat. No. 3,079,272 describes the use of 4 to 5 percent by weight of anionic compounds, such as stearic acid, in "melt-form" in developer compositions containing particulate toner particles to "improve the triboelectric charge relationship" between the toner particles. However, it has been found that when fatty acids, such as stearic acid, are in the toner formulation it is difficult to impart a high net positive electrical charge to the toner particles with the magnetic carrier particles. Stearic acid also decreases the adhesion of the toner particles to paper.
Other useful charge control agents are the non-surfactant, short-chain, quaternary ammonium salts described in Jadwin et al. U.S. Pat. No. 3,893,935 and the alkoxylated amines described in Jadwin et al. U.S. Pat. No. 3,944,943. These quaternary ammonium salts and alkoxylated amines provide high, uniform net electrical charge to a toner powder without reducing the adhesion of the toner to paper. They are not, however, as effective as would be desired over a wide range of relative humidity.
Merrill et al. U.S. Pat. No. 3,694,359 issued Sept. 26, 1972 describes a dry toner containing a wetting agent including certain esters of sodium sulfosuccininc acid. The purpose of the wetting agent is to aid in promoting even, uniform contact between the toner and the paper support to which the toner image is ultimately fixed by heat. No indication is given that its addition serves to control the charging characteristics of the toner particles.